With the rapid development of China's packaging industry, people's understanding of corrugated boxes has determined the quality of cartons from the sense of hand, and gradually improved to use various professional instruments to detect various physical performance indicators. The performance of corrugated boxes is mainly determined by the layers of base paper that make up corrugated board. Therefore, the physical strength of the base paper has become the focus of corrugated packaging companies.
This paper will combine the papermaking process of the base paper to briefly describe the physical properties of the base paper, so that the counterparts in the packaging industry can have a comprehensive and in-depth understanding of the physical properties of the base paper.
Effect of base paper fiber structure on strength of corrugated board
The fiber structure of the base paper is different, and the burst strength and edge strength of the corrugated board produced may vary. The base paper fiber structure is mainly the ratio of raw pulp and recycled pulp, wood pulp fiber and straw pulp fiber in the production of base paper.
All wood pulp kraft paperboards are mostly produced by mixing coniferous wood with hardwood pulp or slow-mixing forest wood pulp and fast-growing forest wood pulp. The proportion of wood pulp varies, and the physical properties of the base paper vary.
The difference in the ratio of the dough to the base pulp of the kraft boxboard will directly affect the resistance of the base paper. Some manufacturers reduce the amount of noodle to save costs. For example, the 175g/m2 noodle kraftliner (Grade A paper) should have a total amount of 60-65g, and only 50-55g in the test. Therefore, the burst value and the ring pressure strength will be low.
In corrugated base paper or ordinary noodle boxboard paper, the content of wood pulp fiber will also affect the ring pressure strength of the base paper.
Relationship between tightness of base paper and strength of corrugated board
Whether it is cardboard paper or corrugated paper, the tightness is an important aspect that affects the strength of corrugated board. The calculation formula of tightness: tightness (g/cm3)=quantity (g)÷[thickness (cm)×10000cm2] (Note: thickness is measured by thickness gauge, refer to GB6547-1998 standard) The tightness of the base paper is the weight of the base paper in the unit volume, and the higher the weight, the greater the fiber density of the paper. From the actual production and testing, the breaking strength is higher. The higher the tightness, the higher the stiffness of the corrugated base paper. When processed into corrugated cardboard, the crucible is formed well and the flat compressive strength is also high (ie, the hardness is better). In practice, it has been proved that the higher the tightness of the paper, the ring pressure strength does not increase, so it has little effect on the edge strength of the paperboard.
The pressing effect of the base paper in the production process, the strength of the tightness will affect the degree of tightness between the fibers and thus affect the ring pressure strength and tensile strength of the base paper, and ultimately affect the compressive strength of the carton, and due to the paper Insufficient tightness will also affect the moisture absorption of the carton, causing the carton to absorb excessive moisture during use or storage and reduce the compressive strength of the carton itself.
The relationship between the sizing degree of the base paper, the moisture and the strength of the corrugated board
The sizing degree and moisture of the base paper have a great influence on the strength of the corrugated board. Whether it is boxboard or corrugated base paper, the higher the sizing degree, the smaller the moisture content changes. On the contrary, the greater the change in moisture content.
When the sizing degree is too high, the base paper fiber will become crispy. Although it has a good performance on the flat compressive strength of the corrugated board, it will affect its burst strength and impact strength. In particular, the folding resistance of the cardboard will be poor, and the indentation fracture will occur after the cardboard is put into a box. The base paper with too low a sizing degree easily absorbs moisture in the air, making the base paper moisture high. The base paper fiber swells after absorbing moisture, and the paper becomes soft. Its resistance to breakage, ring pressure and other strength will be reduced and affect the relevant physical properties of corrugated board.
Base paper ring pressure
The ring pressure test of the base paper is divided into the longitudinal ring pressure strength and the lateral ring pressure strength. It is necessary to pay attention to the ring pressure test sampling. The longitudinal ring pressure strength sampling needs to be consistent with the paper web width direction, and the lateral ring pressure strength sampling. It needs to be consistent with the length of the web. During the test, we will find that although the arrangement of the fibers of the long net machine tends to be disordered, the transverse loop strength will always be lower than the longitudinal loop strength, and as the 0201 type. In the case of corrugated boxes, the lateral compressive strength is consistent with the direction of the corrugated force.
The long-end paper machine increases the amount of fiber tolerated during the transverse ring pressure test while performing the disordered arrangement of the fibers, thereby enhancing the transverse ring-shaped compressive strength of the base paper, and at the same time enhancing the firmness between the fibers due to the disordered arrangement of the fibers. The degree, in turn, ensures the stability of the compressive strength of the corrugated box, so that the drop in the resistance value between the individual cartons is reduced.
Base paper burst strength
The base paper burst strength refers to the uniform maximum pressure that the paper or paper can withstand per unit area perpendicular to the surface of the sample.
When making paper, the ratio of fibers in the pulp tank has a great influence on the bursting strength. The more the ratio of the wood pulp in the coniferous forest, the larger the proportion of long fibers and the higher the burst strength. On the other hand, if more waste paper is recycled, the lower the bursting resistance.
There is a saying in the paper industry: "three-point papermaking, seven-point beating." The bursting resistance and folding strength and ring pressure strength of the carton board are different depending on the beating effect. The fiber length is kept as much as possible during the beating, and the fiber is fully absorbed and swelled, less cut, multi-twisted, and the specific surface area of ​​the fiber is greatly increased. And the amount of free hydroxyl groups, when the paper is formed, the fiber arrangement is disordered, and the index such as the burst strength can be remarkably improved.
In addition, another major factor affecting the burst strength is the bond between the fibers. It is generally possible to add reinforced starch, especially tapioca starch, to the pulp to increase the burst strength of the base paper.
Base paper tensile strength
The tensile strength of corrugated paper depends on the fiber structure of the base paper and the adhesion between the fibers. The tensile strength not only affects the tensile strength of the base paper during use, but also affects the carton after molding. Compressive quality.
If the ring pressure of the paper is good, and the crack length and the tensile strength are not good, the deformation of the carton during the compression test is often large, and the final deformation is like an accordion. This is because of the texture and structure of the fiber. Or the connection strength between the fibers is poor, and when the carton is subjected to the compression test, the fiber at the edge of the crimping line is first stressed, and the loose and weak fiber does not well convey the force received by the carton to the whole of the carton, with Partial collapse of the edge, and the force part gradually moves toward the middle of the box, forming a test phenomenon such as an accordion. If the tensile strength is good, then this will not happen, and the carton will be subjected to a lot of pressure with a small amount of deformation.
A misunderstanding of physical performance
To determine whether the quality of paper is not determined by a single physical indicator, the physical properties of paper should be a comprehensive indicator. In order to prevent the carton enterprises from cutting corners, the carton users put forward the requirements for the compression index of the carton, and put forward the excessive weight requirement for the matching of the base paper. However, in order to meet the requirements of the customers, the carton enterprises only increase the weight of the paper. Reduce paper strength to ensure cost.
This requirement of the customer has already rejected the low-weight and high-strength corrugated base paper. In fact, considering the comprehensive performance, the low-weight and high-strength corrugated base paper is much more stable than the low-strength and high-weight corrugated base paper, and Since the stability of the base paper reduces the difference between the individual physical properties of the carton, the safety of the use of the carton is increased, and the paper of low strength and high weight is difficult to achieve this effect.
Comprehensive control measures for physical properties of base paper
1. Make adequate adjustments to the raw material fiber structure, pulp and paper process, production capacity and technology, quality control capability of the raw paper supplier, and select a stable supplier as a qualified supplier.
2. Formulate the original paper inspection standards and acceptance specifications. Co-confirm with the supplier as the basis for the acceptance of the contract, and the original paper that does not meet the requirements of the allowable range may be treated by downgrading or returning. In particular, it is strictly controlled on the resistance index, ring pressure strength, banner weight and moisture detection. The scope of non-compliance with the requirements of the indicator shall not be accepted as qualified raw materials.
3. Strengthen the inventory management and protection measures of the base paper. First, the ventilation of the warehouse is good and the moisture is not controlled, and the water deviation of the base paper is controlled. The second is to avoid the falling of the original paper handling process, flattening or flat core to ensure the tension balance of the base paper during use. The third is to be as advanced as possible, so that the original paper will not be physically reduced due to storage for too long.
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